Soluble Ziegler-Natta catalysts, which are often referred to as metallocenes or single site catalysts, are the most commonly employed catalysts in the commercial .alpha.-olefin polymerization processes. This is especially true for the production of polyethylene or ethylene/.alpha.-olefin copolymers. Metallocenes are bis(cyclopentadienyl) metals, which generally possess a sandwich structure in that the metal is sandwiched between the two cyclopentadienyl groups (both cyclopentadienyl groups bind with the metal atom in .pi. fashion).
A conventional Ziegler-Natta catalyst typically contains a mixture of titanium, vanadium, and/or aluminum complexes. Several shortcomings, however, have been observed with the conventional Ziegler-Natta catalyst in the polymerization or copolymerization of ethylene/.alpha.-olefin. These include: relatively broad molecular weight distribution, inadequate light transparency, high content of extractable substance, and relatively low monomer concentration in the polymerization process. The potential use of soluble Ziegler-Natta catalysts to form single active site catalysts for olefin polymerization was first suggested by Breslow and Newburg in J. Am. Chem. Soc., vol. 81, pp81-86 (1959), in which a mixture of a metallocene, bis(cyclopentadienyl) zirconium dichloride, and aluminum alkyl was used as the catalyst. However, the catalyst activity of the single site modified Ziegler-Natta catalyst disclosed by Breslow and Newburg was inferior to the conventional Ziegler-Natta catalysts. It was not until 1980, when Sinn and Kaminsky reported in Advances in Organometallic Chemistry, vol. 18, P123 (1980), in Germany Patents 2,608,933 and 2,608,863, and in European Pat. No. 35242, that bis(cyclopentadienyl) zirconium dichloride can be effectively used, with aluminoxane as a cocatalyst, in the ethylene polymerization processes.
With respect to the use of bis(cyclopentadienyl) metallic compounds as polyethylene catalysts, European Patent App. No. 129368 (1984) ("Eur-368") discloses a catalyst composition comprising bis(cyclopentadienyl) zirconium dichloride as a primary catalyst and methyl aluminoxane as a co-catalyst for ethylene polymerization. The molecular weight of polyethylene obtained using the Eur-368 catalyst can reach 140,000, with a molecular weight distribution of 3.5. European Patent App. No. 128045 (1984) ("Eur-045") discloses a catalyst composition comprising a bis(cyclopentadienyl) zirconium dialkyl and a bis(cyclopentadienyl) titanium dialkyl catalyst for ethylene polymerization. The molecular weight of the polyethylene polymer that can be obtained using the Eur-045 catalyst was increased to 323,000; however, the molecular weight distribution was also increased to 5.51. European Patent App. No. 260999 (1988) ("Eur-999") discloses a catalyst composition comprising bis(n-butylcyclopentadienyl) zirconium chloride as a primary catalyst and methyl aluminoxane as a co-catalyst for ethylene polymerization. The molecular weight of polyethylene obtained using the Eur-999 catalyst can reach 185,000, with a favorable molecular weight distribution of 1.9. However, the Eur-999 catalyst has an activity of only 461 gPE/mmole Zr.multidot.hr. European Patent App. No. 226463 (1987) ("Eur-463") discloses the use of bis-(cyclopentadienyl) titanium methyl chloride as a catalyst for ethylene polymerization.
The above examples involved the use of methyl aluminoxane as cocatalyst. The use of a bulky boron-containing anion instead of aluminoxane as a co-catalyst or active agent in ethylene polymerization was first disclosed in European Patent App. No. 277003 and 277004 (1988) ("Eur-003" and "Eur-004"), in which tributylammonium tetra(pentaflurophenyl) borate and 7,8-dicarborane were used as a co-catalyst.
Additionally, Japanese Patent App. No. 63218707 (1988) ("Japan-707") discloses a catalyst composition for ethylene polymerization; it comprises bis(cyclopentadienyl) titanium dichloride as a primary catalyst and the co-catalyst contains methyl aluminoxane and 1,2-dichloroethane. The Japan-707 catalyst was able to improve the catalyst activity in ethylene polymerization to 21,400 gPE/mmole Ti.multidot.hr. More recently, European Patent App. No. 384171 (1990) discloses that by using bis(cyclopentadienyl) titanium dichloride and ethyl aluminoxane as catalysts, the catalyst activity in ethylene polymerization can be further improved to 131 KgPE/gTi.multidot.hr. In World Patent Application No. WO9109882 (1991), it was disclosed that the molecular weight of polyethylene can be increased to 594,000 with a molecular weight distribution of 2.15, using a catalyst composition that contains bis(cyclopentadienyl) dimethyl chromium and dimethylaniline-tetra(pentafluorophenyl) borate. U.S. Pat. No. 5,258,475 discloses the use of a catalyst composition for ethylene polymerization which contains bis(cyclopentadienyl) zirconium dichloride, aluminum trimethyl (i.e., trimethylaluminum), and tributyltin oxide.
Other catalysts have also been disclosed in the prior art which are organometallic compounds containing bis(cyclopentadienyl) and aromatic rings such as indene or fluorene as ligands. In European Patent App. No. 303519 (1987), it is disclosed a catalyst composition containing diindenyl zirconium dichloride, methyl aluminoxane and tetraethyl silicate for use in the copolymerization of ethylene/1-hexene; the catalyst composition exhibited a catalyst activity of 16,800 gPE/gZr.multidot.hr. The two indene molecules can be linked together with carbon or silicon atoms, such as the dimethylsilanediyl group (Me.sub.2 Si), whose use was disclosed in U.S. Pat. No. 4,871,705 (1990), or the isopropyl group, whose use to provide such linkage was disclosed in European Patent App. No. 413326 (1991).
Examples of using fluorene compounds, which contain three rings, as catalyst in ethylene polymerization include those disclosed in European Patent App. No. 530908 (1993), in which bis(cyclopentadienyl) and a fluorene group are .pi.-bonded to a metal atom (zirconium); in European Patent App. No. 528207 (1992), in which an indene group and a fluorene group are .pi.-bonded to a metal atom (zirconium). In both disclosures, the isopropyl group is used to provide the necessary linkage. In Canada Pat. No. 2,067,525 (1992) ("Can-525"), two coordinating fluorene groups are linked to a zirconium compound via an ethylene group to prepare a catalyst for use in ethylene polymerization. With the catalyst disclosed in the Can-525 patent, the molecular weight of the polyethylene prepared can reach 71,900, with a density of 0.97. European Patent App. No. 566988 (1994) discloses the use of fluorene zirconium compounds in the ethylene/5-norbornene copolymerization.
A number of organometallic catalysts containing only mono (cyclopentadienyl) as ligands have also been disclosed. These include the catalyst composition, which contains C.sub.5 Me.sub.4 SiMe.sub.2 NC(CH.sub.3).sub.2 ZrCl.sub.2 and methyl aluminoxane, as disclosed in European Patent App. No. 416815 (1991) for use in the copolymerization of ethylene and 4-methyl-1-pentene or 1-hexene; and the catalyst composition, which comprises CsMe.sub.4 SiMe.sub.2 NC(CH.sub.3).sub.2 ZrMe.sub.2 and PhNMe2H.sup.+ B(C.sub.6 F.sub.5).sub.4.sup.-, as disclosed in WO 9200333 ("WO-333") (1992). With the catalyst composition disclosed in WO-333, the molecular weight of polyethylene can reach 900,000. In U.S. Pat. No. 5,214,173, it is disclosed a catalyst composition for the polymerization of polyethylene which contains (C.sub.5 Me.sub.5)(C.sub.2 B.sub.9 H.sub.11)ZrMe and triisobutyl-aluminum; the molecular weight of polyethylene produced was 182,000.